Telephony



June 8 1926.

TELEPHONY Filed Nov. 5,

F. LOWENSTEIN 3 Sheets-Sheet 1 f Lawenatez'n 9 61cm neg June 8 1926.

F. LOWENSTEIN TELEPHONY Filed Nov. 5, 1920 3 Sheets-Sheet 2 June 8 1926.

F. LOWENSTEIN TELEPHONY Filed Nov. 5, 1920 3 Sheets-Sheet 5 gnvm 1km Z'Lowensiain Patented June 8, 1926.

UNITED STATES PATENT OFFICE.

FRITZ LOWENSTEIN, OF BROOKLYN, NEW YORK; JOHN C. WAIT, ADMINISTRATOR OF SAID FRITZ LOWENSTEIN, DECEASED,

ASSIGNOR, BY MESNE ASSIGNMENTS, TO

RADIO- PATENTS CORPORATION, A DOMESTIC CORPORATION.

TELEPHONY.

Application filed November 5, 19%0. Serial No. 422,035.

This invention relates to telephony," and is especially advantageous in connection with radio telephony. The invention comprises a method of and apparatus for transmission of speech or other form of intelligible sound. As a description of the invention when employed in radio telephony will render obvious its applicability to wire telephony, the invention will be explained more particularly hereinafter in connection with radio telephony for the sake of a concrete example of one way in which the invention may be practiced.

Notwithstanding the great advances made in radio telegraphy, the art of radio telephony has not progressed heretofore materially beyond the experimental stage. The problem of transmitting intelligible speech or other sound by means of radiant energy is beset with much greater practical clifficulties than is radio telegraphy. In the latter, it suffices to transmit code signals that can be detected by a suitable receiver; the received signals, though possessing a predeterminable pitch or tune if rendered audible in a telephone receiver, yet having no necessary relation to sound produced at the sending station. In wireless telephony, on the other hand, it is essential that the sounds originating at the sending station be reproduced in suchform at the receiving station that they are at least recognizable by and intelligible to the car as such. These requirements at once impose certain definite restricting limitations on the methods and .apparatus to be employed.

For radiotelephonie work it is desirable to employ undamped or continuous oscillations. Of the various meansavailable for producing undamped oscillations, the thermionic devices of the vacuum tube or audion' type have offered greatest promise of success; but inability in actual practice to attain more than relatively small power output with devices of this character has been heretofore an apparently insuperable obstacle to wireless transmission of intelligible speech except for relatively short distances.

I have observed that, in these prior attempts at radiotelephonic communication, a very large proportion of the available trans mitting energy is consumed in the effort to amplify and radiate oscillations which include the entire range of frequencies characterizing the original sound; whereas I have found that thls is not essential to the intelligibility of the received message. Ac-

cording to the present invention, therefore,

the difficulties heretofore encountered are to a great extent overcome, and the efficiency and practical range of radiotelephomc communication are much increased, by radiating and receiving oscillations corresponding to only a portion of the Wave frequencies characterizing the voice or other audible sound to be communicated to a receiving with or Without restoration of some or all of the untransmitted wave frequencies comprised in the composite of frequencies representing the original sound. In the case of Vocal sound, for example, the relatively low frequency sound Wave components consume a disproportionately large amount of energy in transmission although they are the least essential to intelligibility. In practice, therefore, I find it desirable to suppress or sift out for the most part the relatively low frequencies, and to utilize substantially all the available transmitting energy to send the relatively higher frequencies. In other Words, the bulk of the oscillations transmitted and received in accordance with the invention correspond to those of a voice or other sound-produced current, in which the amplitude of the higher audio frequency components is exaggerated. Thus, by a comparatively slight modification of radiotelephone systems heretofore proposed, it is possible greatly to increase the operating range and to attain results that are of the greatest practical value.

With this introductory outline of the principles underlying the invention, a detailed description of a method and apparatus embodying said principles in practical form will now be given.

In the accompanying drawings which illustrate such method and apparatus,

Fig. 1 represents diagrammatically an arrangement of circuits characterizing a relatively simple form of radiotelephonic transmitter system embodying the invention;

Fig. 2 illustrates diagrammatically an em bodiment of the invention in a more complex transmitter system; and

Fig. 3 illustrates diagrammatically a combined transmitter and receiver system within the invention.

Referring first to the simple form of system illustrated in Fig. 1, adapted in this instance for transmitting at one wave length only for a given adjustment, X designates generally telephone microphone transmitter circuit means; Y' designates generally transmitting circuit means, such as amplifying and radiating circuit means; and Z designates generally what may be termed discriminating or sifting out tuned oscillatory circuit means interposed between and coupled to both X and Y. Said circuit means Z is current operated through its coupling to X; and its characteristics are such that it respends chiefly and most readily to oscillations of frequencies higher than say 700 or 800. Oscillations set up in Z, including audio-frequencies derived from X but modified to suppress to a greater or less extent the lower frequencies, are superimposed upon high-frequency oscillations generated in Y, the resultant modulated high-frequency oscillations being radiated in any known or suitable manner.

It will be observed that in the described arrangement, Z functions to transfer from X to Y oscillations of frequencies which for the most part lie in the upper part of the entire range of audio-frequency oscillations set up in X by the voice or other form of sound; while most of the low-frequency oscillations are suppressed or sifted out by the discriminating circuit means Z.

Speaking broadly, it was common practice priorto the present invention to couple- X and Y (or other equivalents) directly so that all oscillations of the voice current set up in X were impressed indiscriminately upon the high frequency oscillations in the amplifying and radiatin circuit means, Y. My invention here speci cally shown, in so far as concerns transmitter means. consists in the interposition between X and Y, or their equivalents, of the selective or discriminating circuit means Z which sifts out or suppresses most of the lower frequencies and allows to pass on to Y substantially only the higher frequencies which principally determine intelligibility. The energy of amplification and radiation being concentrated on the higher voice frequency oscillations to substantial exclusion of the lower frequencies, a material increase in transmitting range results.

The specific form of circuit means X and Y is not material to the invention so long as is operative to produce audio-frequency oscillations properly representative of the sound to be transmitted, and is operative to amplify and radiate oscillations impressed upon it from X by way of Z. Numerous arrangements of circuits satisfying these requirements are known in the art, although heretofore their practical utility has been comparatively limited as hereinbefore pointed out. Different typical arrangements for circuit means X and Y are shown in Figs. 1, 2 and 3.

The selective or discriminating circuit means Z may also vary widely in form, but is essentially an oscillatory circuit tuned or tunable to transfer from X to Y oscil lations of only a predetermined portion of the frequency range characterizing the voice or other audio-frequency oscillations set up in X. For the sake of simplicity the same form of circuit means Z is shown in all figures of thedrawings, and comprises inductances 10, 11, and capacity 12. One or more reactance elements of the circuit may be adjustable for tuning, and in this instance the condenser 12 is shown adjust-able. -The inductance winding 10 constitutes the secondary of an induction coil, of which the primary 13 is in the telephone transmitter circuit X, the latter including also the battery 14 and microphone transmitter 15.

The amplifier of the Y circuit means comprises in this instance a single vacuum tube device 16, of which the filament is heated by the usual A battery and is connected to an intermediate point in the B battery a part of which is in the plate circuit. As shown, the grid is most desirably connected to a point in the B battery more negative than the filament. Inductance 17 in the grid circuit constitutes the secondary of a step-up transformer, of which 11 is the primary, coupling the selective or discriminating circuit Z to the amplifier. The grid circuit is loosely coupled to the plate circuit as indicated at 18. The amplifier is coupled to the antenna circuit 19 by the oscillation transformer 20-21, as shown. The conplings 11--17, 18, and 20-21 may be variable, as shown.

The system illustrated in Fig. 2 is also adapted for transmitting at but a single wave length without readjustment of the circuits, but it embodies further details of circuit'design which may advantageously be employed. The inclusion in the description hereinafter of specific electrical values for various elements in the system is to be understood as for the purpose of illustration only, and not as restrictive.

The Z circuit is the same as before, and the broad underlying principles of operation are unchanged. The antenna circuit comprises the antenna 22, the antenna coil 23, and variable condensers 24 connected in parallel, as shown. The antenna circuit may be tuned to any desired wave length by adjusting the connection 25 to the COll or (and) by suitably setting the condenser switches 26. The output or plate side of an oscillatory tube 27 is adjustably coupled to its input or grid side through the tuned antenna circuit by means of a lead 28 whose terminal 29 is movable over suitable coupling taps on coil 23, and a second lead 30. By means of this coupling, undamped high frequency oscillations generated in the oscillator tube circuit are impressed upon and radiated by the antenna circuit. The coupling for maximum power delivery to the antenna can be found by shifting the contact terminal 29 over the coil taps until a maximum reading is obtained in the ammeter 31 in the antenna circuit. The required W21; length is obtained by adjusting the position of the variable connection 25 to give the proper value of inductance in the antenna circuit. The setting of the variable condensers 24 also affects the wave length and also supplies a voltage to the grid of the oscillator tube 27, depending on the capacity represented by such setting. As this voltage must be of definite value to make the tube oscillate at the required wave length, the variability of the antenna circuit capacity is important to enable adjustment to give the proper voltage for each wave length. The oscillator tube 27 is energized by a 350 volt D. C. generator (dynamotor) 32, the positive lead from which is connected through a switch 33 and milliammeter 34 to the plate of said tube. A retardation coil 35 (say 3 M. H.) is inserted in this connection to prevent the generator shunting the high frequency current in the condenser 24. A condenser 36 (say 500 M. M. F.) in the grid coupling lead keeps the positive plate voltage off the grid. A resistance 37 (say 10,000 ohms) provides a leakage path for the charges that would otherwise accumulate on the grid, and is so proportioned as to use this leakage current to maintain a negative potential of a suitable value on the grid. A retardation coil 38 (similar to 35) in series with said resistance and a condenser 39 (750 M. M. F.) in parallel therewith, prevent flow of high frequency current in the grid leak resistance and serve to diminish the internal high frequency losses in the set.

The high frequency current generated by the oscillator tube 27 is varied or modulated in accordance with the voice current by vary ing the power delivered to said tube. In the present instance this is accomplished by means of another vacuum tube generating voice frequency current and associated with the oscillator tube in a manner now to be described. The retardation coil 40 (1% H.) is connected in series with the generator 32, and the plate circuit of the modulator tube. 41 is connected in parallel with the plate circuit of the oscillator tube 27 across the generator leads on the oscillator side of the retardation coil 40. The modulator connected in this way acts as a speech-operated resistance. The higher frequencies of voice current set up in the transmitter circuit X by speech into the microphone transmitter 42, are superimposed upon the circuit Y by the shifting circuit Z interposed between .13 and 17', said circuit Z functioning to suppress or eliminate low audio-frequencies, as in the system of Fig. 1; the voice current voltage being stepped up by transformer 11, 17 and impressed upon the grid of the modulator tube 41, the resistance of this tube being thus varied in accordance with variations of the voice current. Since the retardation coil 40 tends to keep the current from the generator 32 constant, the variation in the resistance of the modulator tube varies the current through the latter and causes the current through the oscillator 27 y to vary inversely. This variation of the power through the oscillator tube causes it to deliver to the antenna radio-frequency waves modulated by the superimposed oscillations of higher voice frequencies. The retardation coil 35 connected to the plate of the oscillatory tube serves to prevent high frequency currents from flowing into the plate circuit of the modulator tube. Besistances 43 and 44, which can be placed in shunt by switches 43 44, with the filaments of the oscillator tube and modulator tube, respectively, are provided to permit employment of tubes of differing characteristics, when desired. An ammeter 45 and knife. switch 46 are included in the circuit containing the oscillator and modulator tube filaments and the A battery. In the present example, this battery circuit also includes the filaments F of three vacuum tubes in a receiver system to which the antenna may be alternatively connected by a relay switch indicated at 47, which receiver system will be referred to further hereinafter in connection with Fig. 3.

With the transmitter system shown in Fig. 2, it is possible to transmit on one wave length only without readjusting. Fig. 3 illustrates a transmitter system similar to that of Fig. 2 in all essential respects except that lit) it provides for selective transmission at any actuating means indicated diagrammatically in dotted lines at 55 for movement in unison. There are five positions for the combination of switch devices shown, four connections being made simultaneously for each position. By preliminarily connecting the proper inductance and capacity in the antenna circuit and obtaining proper coupling for each wave length, the apparatus may be set so that the transmission at any one of five wave lengths can be had simply by operating the common switch-actuating means 55 to move the three switch devices into the corresponding position. In order to obtain the proper Wave length and coupling for each of the five positions, the preliminary setting for each position requires separate adjustments on the antenna coil 56 of the two adjustable taps connected respectively to the pair of contact points in sets 53, 54, (one point being in each set) corresponding to that particular wave length. The capacity of the corresponding input condenser 26 isalso adjusted for each position of the switch; and the flat spiral inductance 57, which is connected in series with the antenna coil, is also adjusted for each position of the switch to obtain a finer adjustment of wave length than is afforded by the adjustable taps on the antenna coil.

It will be noted that the negative grid voltage for the oscillator and modulator tubes and also the voltage for the telephone transmitter is supplied by a suitable drop of potential alongthe filament circuit.

The receiver system, to which the antenna 22 may be alternatively connected by relay switch 47, comprises an antenna circuit in which the modulated high frequency waves sent out by a transmitter such as that herein shown are received and'amplified by resonance, a means for detecting the current set up by these oscillations, and a means for amplifying the voice current component so that it will be heard in the telephone receiverv58. The antenna circuit comprises the antenna 22, the antenna coil consisting of two inductively coupled windings 59, 60, and a variable air condenser 61 (max. 850 M. M. F.) The primary winding 60 has four taps connected to the switch 62, by means of which the amount of winding 59 in circuit can be varied. The secondary winding has two taps connected to points 63, 64, engageable by one of the arms of the double arm dial switch 65. The tap connected to contact 63 is adjustable, as shown. Opposite to contact 63 is contact 63, and opposite to 64 is 64. Another contact 66, connected to the antenna circuit between condenser 61 and winding 59, is opposed on the switch dial by a dummy or open circuited contact 66. When the switch connects points 66,66, the secondary windin is cut out so that the antenna circuit can be tuned only by operating switch 62 to vary the primary inductance, and by adjusting the variable condenser 61. When the set is connected in this way, the antenna circuit may be tuned to wave lengths of from 200 to 600 meters. It may be more sharply turned by connecting in the secondary circuit. When the switch 65 is moved to connect points 63, 63', the set may be tuned to wave lengths of from 200 to 250 meters by shifting the adjustable tap on the secondary winding 60; and when it connects points 64, 64' it can be tuned to wave lengths of from 220 to 600 meters by adjusting the variable condenser 67 which is in series with the antenna coil secondary.

The detector circuit is bridged across the primary of the antenna coil when the switch 65 is thrown to the position 6666 and is directly coupled to the antenna circuit; the detector is bridged across the secondary whenvit is in either of the positions 63-63 or 64.-64', and is inductively coupled to the antenna circuit. The secondary coil is movable with respect to the primary, so that the inductance coupling can be varied as desired.

The. detector comprises the vacuum tube 68 and stopping condenser 69 (.001 mf.), together with a two-megohmresistance 70 grid leak. The detector is arranged to rectify and amplify the voltage impressed on the grid due to the oscillations in the antenna circuit. The stopping condenser increases the rectifying action of the tube, and the grid resistance prevents the accumulation of negative charges on the rid, which if not continually removed, won (1 stop the action of the tube. The resistance 70 is so connected in the circuit as to make the normal potential of the grid slightly positive with respect to the filament and thereby to enable the tube to work under the best con ditions.

The variations of potential to the plate circuit of the detector tube are impressed upon the grid of the first-stage tube 71 of the amplifier. The plate current is sppplied through the retardation coil 72 (15 the inductance of which prevents the loss of audio frequency in the circuit. A 1500 M. M. F. condenser 73 is interposed to keep the plate voltage off the grid of the amplifier tube, and. a one-megohm resistance 74 is inserted between the grid and filament to prevent accumulation of negative charges on the grid. Where necessary or desirable, as in employing a vacuum tube of some other type, the resistances 7a may be shunted by connecting terminal 7 5 to the filament.

The output of the first stage tube 71 is further amplified by the tube 76, which is similarly connected and the output of which goes directly to the telephone receiver. The output of the detector tube is shunted by a small condenser 77 (500 M. M. F.) connected between the plate and the filament to absorb the high frequency component of the rectified current so as to prevent it from passing into the amplifier and interfering with the amplification of the voice current. The resistance 78 (3000 ohms) is arranged so that it can be connected by a switch 79 to decrease the amplification when desired. The plate voltage for the amplifier tube is 45 volts, in a typical instance, and is supplied by two batteries 80, 81. I The filaments of all tubes in the tranmitter-receiver are connected in series and the current for them is supplied by the battery 82. The set is arranged so that when it is required to receive Without transmitting for any length of time, the transmitter tubes need not be lighted.

The set shown in Fig. 3 is normally in receiving position. It is placed in condition to transmit by pressing the push button 83 Which operates relays 84, 84. and 84" to connect the antenna to the transmitter, the plate voltage to the transmitter, and the voltage to the telephone transmitter circuit. It also disconnects the voltage from the amplifier and the antenna from the receiver.

If it be desired to supplement the received oscillations by adding thereto relatively low frequency oscillations corresponding at least in part to those sifted out or suppressed at the transmitting end, this may be efiected in various ways. For example, an alternating current generator 85 may have its circuit coupled at 86 to the circuit which in-- cludes the plate of the tube 7 6 and the microphone receiver 58. Said generator may be driven by the variable speed motor 87 at a speed appropriate to produce currentof any frequency which it is desired to superimpose upon the received oscillations and thus to restore oscillations of a suppressed or sifted out frequency.

What I claim is:

1. Telephone apparatus comprising the combination, with telephone circuit means, and amplifying and transmitting circuit means operable to receive therefrom and to transmit oscillation of audio frequency, of discriminating circuit means interposed between said telephone circuit means and said amplifying and transmitting circuit means and coupled to both, said discriminating circuit means having its constants arranged to transmit principally a predeterminable range of higher audio frequencies set up in said telephone circuit means restricted sufficiently to consume appreciably less power than that required to transmit the full range without destroying the intelligibility of reproduction and a receiving circuit arranged to cooperate with said transmitting circuit, means for reproducing said higher audio frequencies, and means connected with said receiving circuit for restoring at the receiver the lower audio frequencies discriminated against by said transmitting circuit.

2. Telephone apparatus comprising the combination, with a telephone circuit, an amplifier circuit, and a radiating antenna arranged to be energized by said amplifier circuit, of a discriminating. circuit coupled to both the telephone circuit and the amplifier circuit, said discriminating circuit being tuned to transmit from said telephone circuit to said amplifier circuit oscillations of higher audio frequencies in preference to those of lower audio frequencies to an extent sufiicient to efiect an appreciable saving in power over that required to transmit the full range of frequencies and insufficient to destroy the intelligibility of reproduction and a receiving circuit arranged to cooperate with said radiating antenna for reproducing said higher audio frequencies, and means connected therewith for renewing the lower audio frequencies discriminated against at the transmitter.

3. Radio telephone apparatus comprising the combination, with a radiating circuit, oscillatory means arranged to deliver power thereto, and means for setting up continuous alternations of predeterminable frequency in said oscillatory means, of a telephone circuit, and modulating means operatively associated therewith and arranged to superimpose upon current oscillations of said oscillatory means oscillations of audio frequency derived from said telephone circuit, said modulating means being operative to sift out or suppress oscillations of lower audio frequencies while transmitting oscillations of higher au'dio frequencies, to an extent sufficient to effect an appreciable saving in power over that required to transmit the full range of frequencies and insufficient to destroy the intelligibiliay of reproduction and a receiving circuit arranged to cooperate with said radiating circuit for reproducing said higher audio frequencies, and a source of audio frequency oscillations connected with said receiving circuit for restoring the oscillations of lower audio frequencies sifted out at said transmitter.

4. Telephone apparatus comprising the combination, a transmitting system with means for producing a voice current which the amplitude of the higher audio frequency components is exaggerated, of means for transmitting oscillations corresponding to those of such exaggerated components and a receivin system having means for combining sai exaggerated components with locally generated components for reproducing said higher audio frequency components at normalcy.

5. Telephone apparatus comprising the combination, with means for producing a voice current in which the amplitude of the higher audio frequency components is exaggerated, of means for transmitting oscillatlons corresponding to those of such exaggerated components, a receiver arranged to be ener ized thereby and means connected with said receiver for reducing said components to normalcy for the reproduction of sound.

6. Telephone apparatus comprising the combination, with means for producing a voice current in which the amplitude of the higher audio frequency components is exaggerated, of means for transmitting oscillations corresponding to those of such voice current, a receiver arranged to be energized thereby, and means for impressing upon the receiver oscillationsof lower audio frequency corresponding in part to the lower components of the voice current.

7. The method of telephonic communication which comprises setting up audio frequencies in a telephone circuit, selectively amplifying and transmitting, to a distant station, oscillations of the higher audio freuencies thus set up in preference to oscillations of the lower audio frequencies, receiving said oscillations of higher au'dio frequencies and combining the higher audio frequencies thus received with lower audio frequencies for the normal reproduction of sound.

8. The method of transmitting and reoeiving articulate speech by radiant carrier wave energy, which consists in suppressing the modulation of lower frequencies from the radiated wave to an extent of range consuming less power than the whole range but insufiicient to destroy the intelligibility when translated into audible speech receiving the fre uencies not suppressed and restoring to t e receiver the suppressed frequencies for the normal reproduction of speech.

9. The method of transmitting and receiving articulate speech by radiant carrier wave energy, which consists in suppressing those modulating frequencies consuming the most power in modulation, to an extent sufficient to effect an appreciable saving of power but insuflicient to destroy the intelligibility of reproduction and restoring at the receiver the frequencies thus suppressed for the normal reproduction of sound.

10. A wireless telephone system comprising a carrier wave radiating means, a modulator associated therewith, a source of high frequency current for said modulator, a source of voice currents, and means between said source of voice currents and said modulator for distorting the voice currents to distort the resultant transmitted voice modulations into a form consuming a smaller amount of power than that consumed in transmitter undistorted modulations without depriving the transmitted speech sounds of intelligibility and receiving means responsive to the resultant transmitted voice modulations, said receiving means having a source of energy connected therewith for reducing said modulations to normalcy for the reproduction of sound.

In testimony whereof I hereunto aflix my signature.

FRITZ LOWENSTEIN. 

